1. Field of the Invention
This invention relates in general to the field of Piezoelectric Transduction. More particularly, this invention relates to the field of Shear Wave Piezoelectric Transducers by providing an apparatus and method for producing polarized shear waves.
2. Description of the Prior Art
Piezoelectric transducers are well known devices having broad applications in the fields of non-destructive test and evaluation, medical ultrasound and materials characterization. Such transducers are employed in the generation and detection of acoustic waves in various media. Piezoelectric transducers can be used to produce both longitudinal and shear waves, with the latter case being of interest here.
An acoustic shear wave is characterized by particle displacements perpendicular to the direction of wave propagation, and is used in numerous types of piezoelectric resonators and transducers. It has been long recognized that the typical shear wave transducer creates in the test object to which the transducer is affixed a single, fixed polarization shear wave response at a given operating frequency. In order to change the polarization of the shear wave in the test object, a transducer must be physically removed from the test object, and then reoriented onto the test object and subsequently rebonded to it. The well-known requirements of removal, reorienting and rebonding a transducer to a test object are disadvantageous because test results obtained at various polarizations are compromised by the degree to which the bonding procedure is not precisely replicated. The compromising of test results because of the limitations of currently available technology has long been understood as a heretofore unavoidable major limitation and an undesirable cost factor.
Those concerned with the development and testing of ultrasonic test and measurement systems have long recognized the need for a structure which has the advantages of improving the accuracy of the ultrasonic tests by eliminating the uncertainties associated with the rebonding procedure, as well as reducing operational costs and complexities associated with that rebonding process. This structure not only has the advantages arising from the elimination of rebonding, but also has the additional advantages of allowing simultaneous measurement of two (2) polarizations. Heretofore, simultaneous measurement of two (2) polarizations required the use of two (2) separate transducers. This invention does not suffer from that disadvantage.
The present invention fulfills this need by providing a Polarization-Sensitive Shear Transducer structure having a MuSLE electrode formed on a substrate with the substrate orientation chosen such that orthogonal shear-wave polarizations are excited by orthogonal lateral-field excitations, thus forming a body that is placed on a test object thereby allowing selective excitation of polarized shear waves.
Examples of dilithium tetraborate plate resonators and transducers may be found in the following references:
R. W. Whatmore and I. M. Young, U.S. Pat. No. 4,634,913, entitled "Application of Lithium Tetraborate to Electronic Devices," issued Jan. 6, 1987;
A. Ballato and J. Kosinski, U.S. Pat. No. 4,950,937, "Method of Making a Resonator from a Boule of Lithium Tetraborate and Resonator So Made," issued Aug. 21, 1990;
A. Ballato and J. Kosinski, U.S. Pat. No. 4,990,818, "Method of Making a Transducer from a Boule of Lithium Tetraborate and Transducer So Made," issued Feb. 5, 1991;
C. D. J. Emin and J. F. Werner, "The Bulk Acoustic Wave Properties of Lithium Tetraborate," Proceedings of the 37th Annual Frequency Control Symposium, June 1983;
R. C. Peach, C. D. J. Emin, J. F. Werner and S. P. Doherty, "High Coupling Piezoelectric Resonators Using Lithium Tetraborate," IEEE Ultrasonics Symposium Proceedings, October-November 1983;
M. Adachi, T. Shiosaki, H. Kobayashi, O. Ohnishi and A. Kawabata, "Temperature Compensated Piezoelectric Lithium Tetraborate Crystal for High Frequency Surface Acoustic Wave and Bulk Wave Device Applications," IEEE Ultrasonics Symposium Proceedings, October 1985;
A. Ballato, J. Kosinski and T. Lukaszek, "Lateral Field Temperature Behavior of Lithium Tetraborate," IEEE Ultrasonics Symposium Proceedings, October 1989;
A. Ballato, J. Kosinski and T. Lukaszek, "Lithium Tetraborate Transducers," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 38, No. 1, January 1991;
J. Kosinski, A. Ballato and Y. Lu, "Measured Properties of Doubly-Rotated Dilithium Tetraborate (Li.sub.2 B.sub.4 O.sub.7) Resonators and Transducers," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 40, No. 2, March 1993; and
J. Kosinski, "Pure Mode Loci for Dilithium Tetraborate Piezoelectric Resonators and Transducers," Ph.D. Dissertation, Rutgers University, May 1993.
Further, the MuSLE electrode arrangement described herein may also be found in:
Related Application CECOM Docket No. 4924 Kosinski, Ballato, and Lu entitled "Crystal Resonator with Multiply Segmented Lateral-Field Excitation (MuSLE) Electrodes," which has been partially assigned to the same assignee, wherein I am a coinventor.
The term "plate resonator" as used herein is defined as a structure wherein the lateral dimensions are much larger than the thickness and is considered to encompass not only planar plates, but also plates with a curvature that is appropriate to the ultrasonic imaging task being considered, as well as other shapes and geometries.